I'm with Jim, learning to do everything yourself is the only way to go. These older Mercedes are very friendly to work on. They were designed for longevity and ease of service. Not plastic throwaway cars like what they're building nowadays.
The sad fact though, is that most Indy shops and all dealers don't have a clue how to work on these older cars. Your car is older than most of the mechanics doing the work!
On top of that, most car repair shops have absolutely no clue when it comes to diesels. Old car + Diesel is a double whammy.
Additionally, many mechanics these days are not mechanics at all; they're "parts replacers". When something isn't working right, they start replacing parts until it does. They don't know how to troubleshoot or care to learn, they just want to replace parts and charge you for them. They can get away with it, because the average consumer doesn't know the difference.
The only place I would trust, is a diesel mercedes specialist; i.e. a shop that specializes in older diesel mercedes and has been working on them for decades. Go anywhere else, and yours is likely the first one they've ever touched. They're learning on your car, and charging you for the privilege.
